Currently, the deployment of heterogeneous networks (HetNets) is viewed as one of the most cost efficient deployment strategies for wireless communication systems in addressing the growing traffic demands and the expectation for higher data rates. Typical cellular networks today are characterized by non-uniform user and traffic distributions. HetNets complement the macro networks with low power nodes (LPN), such as micro, pico, and femto base stations or relay nodes, which can achieve significantly improved capacity and high data rates.
In wireless communication systems with a heterogeneous network deployment, mobility management is a challenging task. Investigations have been undertaken to evaluate performance of hard handover in orthogonal frequency-division multiplexing (OFDM) based cellular systems, such as 3GPP Long-Term Evolution (LTE) or IEEE 802. System level simulation results have been extensively discussed within standardization, such as 3GPP. Simulation results from these studies show that the careful choice of handover triggers (i.e., Time-to-Trigger and signal hysteresis) for cells of different sizes can lead to substantial reduction in handover failure rate, as well as system and service performance improvements.
In wireless communication systems with a heterogeneous network deployment, it is important to choose the optimal mobility trigger to use when a mobile device or other user equipment (UE) is operating in different types of cells, e.g. different mobility triggers should be used when the UE is operating in a macro cell, as compared with when the UE is operating in a femto cell. Failure to use the optimal mobility trigger in such systems can be more severe than in normal networks featuring uniform deployment of cells. For example, with a large Time-to-Trigger in a macro cell, the handover might be delayed, which means that the communication with the serving base station is very likely experiencing higher loss rate and, thus, a higher probability of radio link failure. In addition, this communication interferes with the micro, pico or femto base station eNodeB (eNB) or low power node (LPN) in uplink, and in downlink, UEs served by LPNs and located close to small cell's borders are heavily interfered with by the transmissions to the UE still connected to the macro base station (eNB). These interference effects are more pronounced and more troublesome for an operator in a wireless communication system with a heterogeneous network deployment, than in a homogeneous network.
Currently, the cell selection and handover functionality in LTE is controlled by the network. While the UE provides measurements to the network (i.e., eNB), the UE is unable to influence or respond dynamically to perform optimized handover decisions. The handover triggers, as well as additional mobility related parameters, such as Layer 3 filtering coefficients and measurement bandwidth, are configured by the eNB and transmitted to the UE via measurement control messages. The UE uses the configured parameters to evaluate the configured events, e.g., the cell reselection is implicitly controlled by the network.
One drawback for this conventional method is that the measurement report from the UE is a L3 filtered value of reference signal received power (RSRP)/reference signal received quality (RSRQ), which does not represent the current real-time link quality as a result of filtering and because there is a time delay between when the UE measures RSRP/RSRQ and when the measurements are available at the serving eNB. This time delay can range from few msecs up to hundreds of msecs. Moreover, the UE, once it has reported RSRP/RRSQ to the network, has to wait during a given time period before the UE can report updated RSRP/RSRQ values to the network. This time period is on the order of a few hundreds of msecs.
In wireless communication systems with a heterogeneous network deployment, more up-to-date real-time measurements, which can reflect the current radio link quality, are needed in order to optimize the handover procedure. One reason for this is that smaller cell size involves more dynamic signal strength changes. It is not a viable solution for the eNB to obtain real-time measurements from the UE since this would result in consuming a significantly high amount of UE resources. Further, as discussed above, the cell selection and handover functionality in LTE is controlled by the network, and although the UE provides measurements to the network (i.e., eNB), the UE is unable to influence or respond dynamically to perform optimized handover decisions.
Considering cell size as part of adapting mobility related parameters leads to mobility performance improvements. More specifically, it has been discussed within 3GPP that different mobility triggers should be used when the UE is located in a large cell, as compared with when the UE is located in a small cell. The UE, however, even with these different mobility triggers, is unable to determine which sets of triggers to use since the UE is not aware of the cell size within its current cell, unless instructed by the network. It is noted that, in 3GPP TS 36.423, the Cell Type IE, which contains the information of cell size (very small, small, medium, large), is included in the Last Visited Cell IE in the UE History Information IE, and is included in the Source eNB to Target eNB Transparent Container IE in the Handover Request message over S1. However, this information (i.e., cell type/cell size) is only exchanged among the networks, and the UE does not have any information on cell type and cell size.
Accordingly, there is a need for a method and device for adjusting radio resource management procedures in wireless communication systems with a heterogeneous network deployment that can improve system and service performance by optimizing handover procedures, decreasing the handover failure rate, and by improving radio resource management such as uplink power control, radio link failure and link adaptation.